The microelectronics industry strives toward fabricating high density circuitry by decreasing the minimum feature size of the components on the chip. This requires high-resolution lithography, the principal technique used in patterning microelectronics circuitry. Over approximately the last 20 years, the industry has migrated to shorter wavelength photolithography as the primary means of scaling the resolution to sustain the progressive demand for smaller features. The wavelength of photolithography has migrated from mid-ultraviolet (MUV) wavelengths (350-450 nm) to deep-UV (DUV) radiation (190-300 nm) and vacuum UV (VUV, 125-160 nm). Likewise, the photosensitive materials used in photolithography, i.e., photoresists, have evolved. MUV lithography employed diazonaphthoquinone (DNQ) and novolak-based resists. These materials offered high performance, but were not extendible to DUV and VUV wavelengths due to their opacity at these shorter wavelengths. In addition, these resists were not of sufficient sensitivity to afford high throughput manufacturing.
In response to the need for new, lower opacity, higher sensitivity materials for DUV imaging, Ito et al. disclosed in U.S. Pat. No. 4,491,628 the development of chemically amplified (CA) resists based on photochemically-generated-acid catalyzed deprotection of an acid labile polymer. That is, for positive tone CA resists, acid labile moieties of the polymer are cleaved by acid catalyzed thermolysis reaction that renders the deprotected form of the polymer soluble in a subsequently applied developer, such as an aqueous base. Thus, an image of the projected patternwise radiation is formed in the resist film after development, which can then serve as an etch-resistant mask for a subsequent pattern transfer step. The resolution obtained is dependent on the quality of aerial image and the ability of resist to maintain that image.
CA resists have been developed for 248, 193, and 157 nm lithography. The theoretical dimensional limit of equal-sized half-pitch features is one quarter of the wavelength, λ (K1=0.25) when NA=1, as the dose applied to the resist is equal to the square of the intensity, and thus the resolution cannot be modulated by any more than λ/4, or a pitch of λ/2. The term “K1”, as used herein, refers to one of the Rayleigh K factors which are process dependent constants as shown in the following equation: R=K1λ/NA. The resolution attainable with each advancing generation of materials has been extended towards these limits through the use of low K1 techniques and high numerical aperture tools. For the latest VUV wavelength being developed for manufacturing, 157 nm, with a very high, but potentially manufacturable NA of 0.95, λ/4 equals about 40 nm. To obtain images below this feature size, either an extension of NA to >1, such as is afforded with immersion lithography, or a non-diffraction limited, non-optical lithography system, such as the so-called next generation lithography (NGL) has to be applied. The most promising of these NGLs are extreme ultraviolet (EUV, sometimes referred to as soft x-ray) or electron beam lithography (EBL).
One barrier to imaging in the sub-50 nm half-pitch regime is a phenomenon known as image blur, which diminishes the integrity of the pattern (see, for example, Hinsberg et al., Proc. SPIE, 2000, 3999, 148 and Houle et al., J. Vac. Sci. Technol B, 2000, 18, 1874). Image blur can be defined as the deviation of the developable image from the projected aerial image. Image blur can be divided into two contributing factors: gradient-driven acid diffusion and reaction propagation. Both factors contribute to blur, but to different degrees and with different temperature dependence.
The first factor contributing to image blur is often referred to as acid diffusion and can be described by Fickian diffusion models for solids. The choice of the photoacid being generated from photoacid generator (PAG) and the mobility in the chosen polymer matrix dictate this factor. The mobility in the polymer matrix is dependent on the comprising chemical functionality of the polymer, the free volume of the matrix, the glass transition temperature (Tg) of the polymer and the temperature and time of baking the steps encountered during the resist processing.
A second contributing factor to image blur is sometimes described as reaction propagation and is best described by Arhenius behavior. Activation energy (enthalpy), volatility of products (entropy), and the availability and concentration of deprotection-reaction-dependent co-reactants, such as moisture, dictate the degree to which the reaction propagates away from the original acid profile.
Since the image blur resulting from diffusion of photochemically generated acid has been determined to be on the order of 10's of nm's and is enhanced by post-exposure baking (PEB), it is extremely difficult to create dense (1:1) device features around 50 nm or less using conventional CA resists. In order to achieve high resolution, high sensitivity and high degree of process latitude, both image blur factors must be eliminated or minimized. Both of these contributing factors can be tempered by the addition of acid-quenchers, or bases, which have been shown to reduce image blur. Additionally, it has been recognized that image blur is temperature dependent. Breyta et al. disclose that appropriate baking conditions can optimize the resolution attainable with CA resists in U.S. Pat. No. 6,227,546. Medeiros et al. disclose a method of fabricating sub-50 nm dense features using a CA resist based on acid labile protecting groups having a low activation energy in U.S. Patent Application Publication No. 2004/0265747 A1.
However, in order to print sub 30 nm features, it is required that a CA resist possesses dissolution properties different from those of the above-mentioned prior art resist so that the image collapsing problem in thin film imaging can be prevented. A CA resist is also required to further reduce the diffusion of the photochemically-generated-acid and, at the same time, maintain the desired sensitivity to consistently print sub 30 nm features. In addition, EUV radiation causes a flare problem that limits CA resist performance. Thus, a CA resist has to possess certain optical properties and dissolution characteristics to reduce flare in printing sub 30 nm images.
In view of the above discussion, there remains a need for an advanced CA resist composition for sub 30 nm dense feature resolutions and a method of performing sub 30 nm imaging.